"It" apparently originated in a void containing and surrounding the, a place referred to in the novel as the "Macroverse" (a concept similar to the later established Todash Darkness of The Dark Tower series). It's real name is Bob Gray or Pennywise (although at several points in the novel, It claims its true name to be Robert Gray) and is christened "It" by the group of children who later confront It. Likewise, It's true form is never truly comprehended. It's favorite form is a clown (with fangs and large claws when it stalks a child) known as Pennywise the Dancing Clown, and It's final form in the physical realm is that of an enormous spider, but even this is only the closest the human mind can get to approximating It's actual physical form. It's natural form exists in a realm beyond the physical, which It calls the "deadlights". Bill comes dangerously close to seeing the deadlights but successfully defeats "It" before this happens. As such, the deadlights are never seen and It's true form outside the physical realm is never revealed, only described as writhing, destructive orange lights. Coming face to face with the deadlights drives any living being instantly (a common device). The only known person to face the deadlights and survive is Audra Phillips, Bill's wife.

It's natural enemy is, another ancient Macroverse dweller who, eons ago, created our universe and possibly others. The Turtle shows up again in King's series The Dark Tower. The book suggests that It, along with the Turtle, are themselves creations of a separate, omnipotent referred to as . The Turtle and It are eternal enemies, however It tells Bill that the Turtle died a few years ago when he "puked in his shell and choked to death on a galaxy or two". It arrived in our world in a massive, cataclysmic event similar to an impact, in the place that would, in time, become Derry, Maine.

It's power is apparently quite vast; during the second Ritual of Chüd, It offers the Losers money, power, and supernatural longevity, or as It put it in the novel ("I can make you (the Losers) gods of the Earth) if they spare It. Of course, It could merely have been bluffing in order to save itself.

Through the novel, some events are described through It's point of view, through which It describes itself as the "superior" being, with the Turtle as someone "close to his superiority" and humans as mere "toys." It describes that it prefers to kill and devour children, not by nature, rather because the fears of children are easier to interpret in a physical form and thus children are easier to fill with terror, which It says is akin to "salt(ing) the meat". It is continuously surprised by the children's victories and near the end, It begins to wonder if it perhaps isn't as superior as it had once thought. However, It never believes that the individual children are strong enough to defeat It, only through "the Other" working through them as a group.

For millions of years, It dwelt under Derry, awaiting the arrival of humans, which It somehow knew would occur. Once people settled over It's dwelling place, It adopted a cycle of hibernating for long periods and waking approximately every 27 years. It's waking spells are marked by extraordinary violence, which is inexplicably overlooked or outright forgotten by those who witness It. It's awakening and return to hibernation mark the greatest instances of violence during its time awake.

1715 – 1716: It awoke.

1740 – 1743: It awoke and started a three-year reign of terror that culminated with the disappearance of over 300 settlers from Derry Township, much like the mystery.

1769 – 1770: It awoke.

1851: It awoke when a man named John Markson poisoned his family, then committed suicide by eating a, causing an excruciating death.

1876 – 1879: It awoke, then went back into hibernation after a group of were found murdered near the Kenduskeag.

1904 – 1906: It awoke when a lumberjack named Claude Heroux murdered a number of men in a bar with an axe. Heroux was promptly pursued by a mob of townsfolk and hanged. It returned to hibernation when the Kitchener Ironworks exploded, killing 108 people, 88 of them being children who were engaged in an .

1929 – 1930: It awoke when a group of Derry citizens gunned down a group of known as the Bradley Gang. It returned to hibernation when the Maine Legion of White Decency, a Northern counterpart to the, burned down an army which was called "The Black Spot." One of the survivors, Dick Halloran, appeared in King's earlier novel, The Shining.

1957 – 1958: It awoke during a great storm which flooded part of the city, and murdered George Denbrough. It then met its match when the Losers forced It to return to an early hibernation when wounded by the young Bill Denbrough in the first Ritual of Chüd.

In the intervening periods between each pair of events, a series of child murders occur, which are never solved. The book's surface explanation as to why these murders are never reported on the national news is that location matters to a news story - a series of murders, no matter how gruesome, doesn't get reported if they happen in a small town. However, the book's implied reason for why the atrocities go unnoticed is far more sinister: It won't allow them to be. In fact, It's power over the town is so absolute that It's death in the second Ritual of Chüd causes an enormous storm that damages the downtown part of Derry.

Although It is seemingly defeated by the novel's end, there are hints in King's later works that It is still alive. Furthermore, It had laid eggs shortly before its defeat and whether or not they were all destroyed is never resolved.

The seven Losers are the children who are united by their unhappy lives, their misery at being the victims of bullying by Henry Bowers and their eventual struggle to overcome It.

William "Stuttering Bill" Denbrough: Also known as "Big Bill", he gets his from his bad problem, which became much more severe after his brother's death; although his mother attributes it to a car accident that occurred when Bill was three, it's implied to be more of a psychological problem than a physical one. His brother George was killed by It in 1957. Bill feels slightly guilty about the murder, because he'd been the one who sent George outside bubbles to play. Ever since George died, Bill has been partially ignored by his parents who also blamed him for his brother's death. Beverly Marsh develops an intense crush on him during their time in the Losers Club. When the group returned to Derry in 1985 they sleep together but do not carry their relationship any further. He is the most determined and resourceful of the Losers and is the one who, both in 1958 and 1985, confronts It in the Ritual of Chüd and eventually destroys It. As an adult he marries Audra Phillips, a successful actress bearing a strong resemblance to Bev. As with other King characters, , , , and numerous others, in 1985, Bill is a famous .

Benjamin "Ben" Hanscom: He was dubbed "Haystack" by Richie, after the professional wrestler . Because of his obesity, he has become a frequent victim of Henry Bowers, who once used a buck knife to try to carve his name into his stomach (he managed an unfinished 'H' before Ben escaped). His father died in a plane crash in the army. He also develops an intense crush on Beverly Marsh and the two leave Derry together after the 1985 defeat of It. In later life, he becomes a successful and sheds his excess weight. His mechanical skills become useful to the Losers, from making two to building an underground clubhouse where Mike and Richie have a vision of It's cosmic crash into the site which would later become Derry, Maine.

Beverly "Bev" Marsh: The only female in the group, Beverly is an attractive, redheaded girl from the poorest part of Derry. She has an abusive father (referred to as her stepfather at one point in the novel) who beats her regularly. She develops a crush on Bill Denbrough and her skill with a is a key factor in battling It. All the boys are described as being fond of Beverly; at some point each has romantic or sexual feelings for her. As a child, her father abused her while using his constant catch phrase, "I worry about you sometimes Bev, I worry a lot." As an adult, she becomes a successful fashion designer, but endures several abusive relationships, culminating in her marriage to Tom Rogan, who sees her as a and disapproves of her chain smoking, using it as an excuse to After a brief liaison with Bill, she subsequently departs Derry with Ben following the death of her husband (who was used by It to nearly kill the Losers).

Richard "Richie" Tozier: Known as "Trashmouth", Richie is the Losers' most lighthearted member, always cracking jokes and doing, which prove very powerful weapons against It. He is "too intelligent for his own good" and channels his boredom in hyper-active wisecracking, to the point of being self-destructive—his flippant remark to Henry Bowers leads to almost getting beaten up by Henry and his friends. His childhood stemmed from his rapid-fire being compulsive and almost subconsciously triggered. He is the most devoted to keeping the group together as he sees 7 as a magical number and believes the group should have no more, no less. In later life he is a successful disc jockey. As the DJ he uses his once-annoying and unrealistic voices as one of his main attractions. Like Ben, he has a crush on Beverly though it is not crucial to the . He has bad eyesight and wears thick glasses as a child, but changes to as an adult.

Eddie "Eds" Kaspbrak: Eddie is a frail whose is . At one point in the story the man who runs the pharmacy told him that he had a and that his medicine is nothing but water. He has a worrying, domineering who, ever since his father died, has used to bully him into caring for her. Eddie is easily the most physically fragile member of the group. Richie calls him "eds", which he hates (as is demonstrated when It bites off Eddie's arm and his dying words are to Richie, who calls him "eds": "Richie, don't call me eds. You know I...I... [without finishing his sentence, "I hate it when you call me that"]"). He is a . When Henry and his friends break his arm and his mother tries to prevent the Losers from visiting Eddie in the hospital, he finally stands up to his mother and tells her that he is no longer the helpless kid she thinks he is. He eventually runs a successful business but is married to a woman very similar to his mother. He is eventually killed by It in the final struggle after using his inhaler to wound It, making him the only direct adult victim of It. He also finds the strength to defend himself from Henry Bowers, eventually deforming and killing him in self defense with a broken bottle, even though in the fight his arm is re-broken in the same spot Henry broke it in a scuffle when they were kids. He bleeds to death in the sewers after his arm is bitten off, ultimately dying in the gang's arms.

Michael "Mike" Hanlon: Mike is the last to join the Losers. He is the only child in the group. When he is racially persecuted by Henry Bowers, the Losers fight back against Bowers in a massive rock fight. Mike is the only one of the Losers to stay behind in Derry (and thus the only one to retain his memory of the events of 1958) and becomes the town . He is the one who beckons the others back when the killings begin again in 1985. His father kept an album filled with photos that were important to Derry's history, including several of Pennywise the Dancing Clown. Through the knowledge he acquires of Derry and It, he becomes an amateur historian of the town. He is seriously wounded by Henry Bowers and nearly dies. He later recovers from his wounds but like the others starts to lose his memory of the experience, and of the other Losers. It was later revealed in Insomnia that Mike continued as a librarian and was the boss of one of that book's primary protagonists in 1993.

Stanley "Stan" Uris: Also known as "Stan the Man", Stan is a skeptical, bookish member of the group. He admits that his family takes a relaxed approach to their faith, rather than practicing it devoutly. Logic, order, and cleanliness are deeply ingrained in his psyche. He is the least willing to accept that It actually exists and relies on logic more than anything else. Stan, much like Mike, is racially persecuted by Henry. As a child his main hobby was . He later becomes a partner in a large -based accounting firm. However, upon receiving Mike's phone call, he commits suicide by slitting his wrists in the bathtub and writing 'IT' in his blood on the wall. He chose death over returning to Derry to face the ancient terror despite being the one to slice the Losers' palms in a . It is also implied in the book that Stan remembers more about the children's encounters with It than the others do, sometimes commenting about the Turtle and other events from his time in Derry, though he claims that he doesn't remember what those phrases mean.